Exchange Bias Enhancement in Ultra-Dense Arrays of Sub-100nm Co/CoO Nanodisks*

We report on high stability and enhanced exchange bias (EB) of ultra-dense (Tb/inch²) arrays of sub-100 nm Co/CoO nanodisks. Exchange-biased nanodisks are grown by electron beam deposition of Co (15nm) using nanoporous anodic aluminum oxide (AAO) templates as a mask [1]. The thickness of the CoO layer, and therefore the EB of the nanodisks, can be tuned by the oxidation time of the disks. We have found that an annealing time of 10 minutes at 250°C in air produces the maximum EB (800 Oe) at 50K. A comparative study with a reference Co thin film is presented, showing that the nanodisks develop larger EB and higher blocking temperatures at each oxidation time. Although magnetic dots suffer lateral oxidation, which is not possible in the film, a simple model that takes into account this effect demonstrates that the exchange energy density in dots must be higher than that in the Co/CoO film. This is an exciting example of how nanopatterning can improve the physical properties of thin film systems.

1-Template-assisted electrodeposition of Ni and Ni/Au nanowires on planar and curved substrates.” J. Guiliani, J. Cadena, and C. Monton. Nanotechnology 29, 075301 (2017).